It’s one thing to write a polished, even gripping, opening chapter (since opening chapters are the ones most frequently workshopped and otherwise shown around, they’re usually the ones that get the most feedback.) It’s another thing to maintain that level for an entire novel.
Unfortunately, structure and pacing are important to editors when they get around to judging the whole book. Editors want to see whether a writer can sustain interest through the long middle parts without letting the pace of the story flag, and whether he or she can avoid getting distracted by unnecessary details and by subplots that go nowhere, and whether he or she can resist the temptation to skip important plot and character stuff and rush the story in haste to get to the end. They also want to see if the writer can in fact end a book effectively — not winding things up too fast, not dragging the end out too long, not failing to supply a sufficiently impressive fireworks show for the grand finale.
In truth, however, messing up the middle of the book is far more likely to be fatal than not being absolutely perfect at the end. By the time the readers have reached the final chapters, they’ve invested enough time and thought in the story that a lot of technical problems with the ending will be forgiven, just so long as the readers don’t feel cheated of something they’ve come to feel is owed to them by the story. Any number of really good writers have wobbled on the dismount, as it were — Tolkien ended the Lord of the Rings trilogy at least three times before finally letting his readers stagger off into the appendices, for example, but most of his readers don’t feel cheated by the excess.
But this is why editors want to see the traditional “three chapters and an outline” even for query letters, and why the first thing they’re going to say, when you eventually get a nibble, is, “Send me the whole book.”